RNA silencing is a novel, major biological mechanism controlling gene expression at the level of messenger RNA and chromatin, triggered by double-stranded RNA. Originally identified in lower organisms, recent results indicate that it also plays a crucial role in mammalian development and cell differentiation. Based on recent evidence that RNA silencing can affect B cell development, and strong indications that its dysregulation contributes to B cell lymphomagenesis, we hypothesize that RNA silencing plays specific roles at various stages of B cell development. The latter is a multi-stage process characterized by a sophisticated ordered interplay of chromosomal loci at which somatic gene rearrangements and mutation occur sequentially, accompanied by phases of cellular proliferation and quiescence and a continuous requirement for survival signals that rescue the cells from apoptosis. Together with the self-renewal capacity of certain mature B cell subsets, some of these features strikingly resemble processes that govern stem cell homeostasis and lineage commitment, known to be prime targets of control by RNA silencing. In the proposed experiments we will introduce targeted mutations into sequential stages of B cell development using Cre/loxP-mediated mutagenesis. Using this approach, we will first investigate the extent to which the RNA silencing machinery is required for proper B cell development and function, including the ability of the cells to execute adaptive immune responses and innate immune functions like antigen presentation to T cells. In the latter context, dendritic cells (DCs) will be included in the analysis. In these experiments we will use a conditional d/cerallele which we have generated, having demonstrated that Dicer is required for RNA silencing in mouse cells. Second, we will investigate whether the RNA silencing machinery is regulated during B cell development, and if so, try to understand the basis of such a control. Finally, we will identify and functionally characterize individual si/miRNAs that are selectively and stage-specifically expressed in B cells and DCs, using bioinformatic target gene predictions as a guide and targeted mutagenesis in mice. Overall, we expect new insights into the control of normal and malignant B cell development and the functional activity of B cells in adaptive and innate immune responses. Given the critical role of B cells in protective immunity, autoimmune diseases and lymphomagenesis, we anticipate this work will have significant medical relevance.